Case Study

Slab
Reinforcement Design for Milton Keynes Theatre

irregular shaped slabs and
openings

automatic meshing techniques

wood armer reinforcement
calculations

Consulting engineers Whitby
Bird and Partners use LUSAS Civil & Structural for a variety of
engineering analysis work including reinforced concrete slab design, frequency analysis of
grandstands, and bridge design. A number of recent projects have made use of the
Wood-Armer reinforcement facilities in LUSAS to optimise slab thicknesses and the
calculation of reinforcement quantities.

Overview

The £22 million Milton Keynes Theatre is a typical
example of how analysis using LUSAS can produce savings in design time and give more
efficient reinforcement arrangements. A number of slabs in the theatre were designed with
the aid of LUSAS. The complex geometry and awkward shaped openings of the 300mm thick
suspended ground floor slab provided an ideal oppportunity for using the automatic meshing
facilty. With this feature, triangular, quadrilateral or irregular meshing can be used -
all the engineer need specify is the number of default elements along each slab boundary.
The 53m x 30m ground floor slab was modelled using thick plate elements each of a
thickness to suit the slab depth at that location. Pinned supports were provided at the
locations of walls and other supports. Dead and live loading, point loads from columns and
onerous loads caused by moveable seating tiers were also specified. A simple linear static
analysis gave results for post-processing use.

Load combinations and enveloping facilities in LUSAS allowed
maximum effects to be calculated for use in Wood Armer reinforcement calculations. By
using the Wood-Armer facilities in LUSAS accurate reinforcement quanties can be obtained.
Reinforcement moments can be calculated for primary and secondary layers in the top and
bottom faces of slabs for any direction of reinforcement.

Contour plots showing design areas of reinforcement
quantities were plotted. The minimum contour value was set to represent the moment of
resistance of 12mm diameter high yield steel bars at 150mm centres which were required
primarily for cracking purposes. Plotting contours for bending moments in the top and
bottom of the slab then showed in a straightforward manner the extent and size of the
additional reinforcement required to resist the bending moments induced. By using the
graphing facilities in LUSAS, localised bending moment diagrams were created to further
aid the detailing process.

Henry Woodlock, the engineer who used LUSAS for the slab design of the
Milton Keynes Theatre said: 'Hand calculations for this type of work are not
straightforward. By using the Wood Armer facilities in LUSAS it is easy to view critical
and non-critical areas of slabs and provide a far more accurate reinforcement
arrangement".

Slab reinforcement design for the Faculty of
Divinity, Cambridge

In another example, the ground, first and second floor slabs of the
Faculty of Divinity at the University of Cambridge were also analysed using LUSAS. Using
DXF data a model of the ground floor slab was created and then subsequently modified to
create the first and second floor models. Reactions, deflections and ultimate moments were
obtained for the floor slab and in this situation the results obtained enabled the
concrete slab thickness to be reduced by 50mm.

"Hand calculations for this type of work are not
straightforward. By using the Wood Armer facilities in LUSAS it is easy to view critical
and non-critical areas of slabs and provide a far more accurate reinforcement
arrangement."

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